He took a long-held interest in athletic training and nutrition
and put it through the lens of his hit book "The
4-Hour Workweek," experimenting with different workout
regimens and diets based on research and drawing his own
conclusions.

This love of undertaking often extreme physical experiments has
continued, and one of his latest interests has been intermittent
fasting. Business Insider recently spoke with Ferriss about his
latest book, "Tribe
of Mentors," and he explained why he undergoes a monthly
three-day fast, with additional longer, supervised fasts.

Note that Ferriss was adamant that you shouldn't try any aspects
of his regimen without asking your doctor about it, and that even
short fasts should not be done carelessly.

D'Agostino specializes in studying ketosis, the process in which
the liver secretes hormones called ketones to help break down fat
as a fuel source when there is not enough blood glucose, the
body's primary energy source.

Ferriss wrote in his 2016 book "Tools
of Titans" that he began intermittent fasting and a ketogenic
diet to help alleviate the energy-draining symptoms of the Lyme
disease he had contracted.

Ferriss' fasting schedule

Ferriss' monthly fast typically
begins on Thursday night and ends on Sunday night. He breaks
it down in detail in "Tools of Titans," but it involves plenty of
walking, ketone supplements, and a small amount of coconut oil
put into tea to ensure that ketosis is accelerated.

He allows himself "non-caloric unsweetened beverages" like tea
and black coffee during this period.

Ferriss uses a finger-prick to measure his blood glucose levels
during this period, aiming for a safe level of ketosis. This is
especially important in his occasional longer fasts.

As you can tell, Ferriss likes to take things to the extremes,
but intermittent fasting can also just last 24 hours and yield
benefits. His friend Jocko Willink, a former Navy SEAL commander
who's also an author and podcast host, has a
monthly 24-hour fast that he adopted after perceiving the
benefits of the practice while in the military.

Ferriss told us that since he began regularly inducing controlled
ketosis, he's "had some very very clear cognitive benefits,"
including more energy and focus.